Various conventional sensors measure the magnitude of a magnetic field. Some such sensors measure magnetic field perpendicular to the body of the sensor (e.g., Hall effect sensors) while others measure magnetic field in-plane (e.g., magnetoresistive sensors). Magnetoresistive sensors (xMR sensors) include Anisotropic Magnetoresistive (AMR) sensors, Giant MagnetoResistive (GMR) sensors, Colossal MagnetoResistive (CMR) sensors, and Tunneling MagnetoResistive (TMR) sensors, among others. Often, xMR sensors are multilayer devices which include a sensing layer that is susceptible to magnetization by an external magnetic field, but does not retain such magnetization when the external magnetic field is removed. A resistance of the xMR sensor changes as the magnetization of the sensing layer is oriented by the magnetic field.
Linear position sensors often use a bias magnet and a sensor device arranged near a target. The bias magnet creates a magnetic field through the sensor device and the target. The soft magnetic target has a large relative magnetic permeability (often larger than 300, preferably larger than 1500 or 4000) so that it is efficiently magnetized by the magnetic field of the bias magnet, and generates a responding magnetic field with field strength proportional to its proximity to the bias magnet. The sensor device can include xMR sensors such as those described above. Often, the target has a geared or toothed outer edge, such that the magnetic field generated by the target fluctuates as teeth and gaps on the surface of the target pass beneath the bias magnet. Where the tooth spacing of the target is known, the speed of the target can be determined by measuring the frequency of the fluctuations in magnetic field measured by the sensor package.
Conventional position sensors must distinguish between a magnetic field caused by the bias field source and the field that is related to the position of the target. Often, the bias field is much larger than the target field, and fluctuations in the magnetic field from surrounding devices can complicate accurate measurement of target position.